We have, in pervious studies, made a discovery that the generalized transducing Salmonella phage P22 recombine with various unrelated bacteriophages including E. coli phages lambda and phi 80. We are currently characterizing a number of such hybrids to determine the extent of homology with the parent phages by means of genetic recombination and electron microscopic heteroduplex method. To understand mechanisms of genetic evolution of bacteriophage we are broadening this investigation to study the following: 1) Physical mapping by DNA-DNA hybridization of the homologous regions between P22, P22-lambda, lambda- P22 and lambda. 2) Genetic and physical mappings of phi 80-P22 hybrids. 3) Regulation of the hybrid phage gene expression between gene clusters derived from evolutionary diverse phages P22 and lambda. 4) Regulation of in vitro transcription of the hybrid phages P22-lambda-and lambdaP22. 5) Transducing characteristics of the hybrid phages P22-lambda, lambda-P22 and phi 80-P22.